This invention relates generally to a method for positioning a tool of a multi-joint robot and more specifically, to a positioning method wherein the position, i.e., the location and the orientation of the tool is given in a reference coordinate system and the actual position of the tool relative to the reference coordinate system and the angular positions of the robot joint axes are linked by a coordinate-transformation system of equations. The parameters of the coordinate-transformation equations are determined by the reference distances and the reference directions of the robot joints. The word"tool" as used herein encompasses all devices which can be considered as the hand part of a robot.
Relatively complex coordinate-transformation equation system must be taken into consideration when positioning a tool in multi-joint robots. The actual position of the tool is calculated from the actual angular positions of the robot joints and from the parameters which describe or represent the geometry of the robot, such as the distances and directions of the joint axes. The position of the tool which gives its location and its orientation is referenced by a coordinate system, the origin of which is usually located at the base of the robot. In order to calculate the corresponding angular positions of the robot joints required to bring the tool to a given position, a predetermined position (relative to the reference coordinate system) and the geometry parameters are input into the coordinate-transformation equations systems. Such a method is known from R. B. Paul, "Robot Manipulators", Chapter 2, The MIT Press, Cambridge, (Mass.) and London (GB), 1981.
However, since the actual robot geometry is not ideal in practice, error inducing problems arise, such as the individual joint axes having incorrect angles relative to each other; and actual distances, directions and/or joint spacings being different from the desired distances. These problems lead to errors in the positioning of the tool. Although these errors could be avoided by modification of the corresponding parameters in the coordinate-transformation equation systems, this solution would require an extremely costly numerical coordinate transformation.